Metal treatment



. Jan. 8, 1963 D. A. 3,0 8

METAL TREATMENT Filed Sept. 4, 1957 2 Sheets-Sheet 1 WIDE MEDIUM NARROWSTEP WIDTH STEP WIDTH STEP WIDTH PLASTIC PLASTIC PLASTIC STRAIN STRAINSTRAIN RESIDUAL RES.- I RES.

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ELASTIC STRAIN DIRECTION OF WORKING INVENT OR DORSEY A. PAUL D. A. PAULMETAL TREATMENT Jan. 8, 1963 2 Sheets-Sheet 2 Filed Sept. 4, 1957 02-880400 Hut/N ZOCDQWFQO mmwwpm Jdjgmwm INVENTOR DORSEY A. PAUL ATTORNEY025M151 mmwEm wmobm mmwzx zk Ed a mmomu I I I n N M nl nl United StatesPatent ()fifice 3,071,847 Patented Jan. 8, 1963 3,071,847 METALTREATMENT Dorsey A. Paul, Erie, Pa., assignor to Kaiser Aluminum &Chemical Corporation, Oakland, Calif., a corporation of Delaware FiledSept. 4, 1957, Ser. No. 681,957 5 Claims. (Cl.29--180) This inventionpertains to the forging of metals. More particularly, the inventionrelates to an improvement in the production of forgings of solution heattreatable aluminum alloys.

During the past few decades, aluminum forgings have found increasedusage in the fiabrication of various structures. One factor which hasplayed an important role in the wide spread usage of aluminum forgingsis that a forging of light weight yet with high strength may beproduced. Hot working of solution heat treatable aluminum alloysfollowed by a suitable heat treatment will produce forgings withincreased tensile, compressive and impact strength, and increasedfatigue resistance.

Common types of forgings which have utilized the desirable properties ofaluminum base alloys are the closed the forgings and hand forgin-gs.Hand forgings, to which this application is particularly applicable, aregenerally used where the required number of for-gings is not sufficientto justify the making of a forging die, for example, in the production\of prototypes, experimental designs, etc. Hand forgings are worked tothe desired shape in simple open dies that impose little or no lateralconfinement to the stock. The stock is worked by manipulation duringrepeated strokes of a hammer or a hydraulic press. The

hand forging may then be reduced to the finished shape by machining.

In the usual production of forgings of solution heat treatable aluminumalloys, the stock after forging is solu 'tion heated, quenched, andsubsequently aged. The

quenching of the forging from the solution heat treat temperature causesresidual stresses to be set up in the forging which in turn will causedistortion of the forging during subsequent machining operations. Sincethe majority of aluminum hand forgings undergo machining to produce thefinal shape, the residual stresses created by quenching present a majorproblem.

It is therefore a primary object of this invention to provide a methodfor eliminating or substantially reducing the residual stresses inforgings of solution heat treat- It is a further object of thisinvention to provide a method for relieving residual stresses caused byquenching of forging-s of solution heat treatable aluminum alloys bycold forging the forgings a critical amount sub sequent to the quenchingoperation.

It is also an object of the instant invention to provide a forgedarticle of solution heat treatable alloy wherein the residual stressescaused by quenching have been eliminated or substantially relieved bycold forging.

These and other objects and advantanges of the invention will beapparent from the ensuing description of the invention.

In the accompanyingdrawings, forming a part of the specification:

FIGURE 1 shows schematically the effect of forging step width on thestrain patterns of hand forgings of a solution heat treatable aluminumalloy that have been subjected to cold forging according to thisinvention;

FIGURE 2 is a perspective schematic view of a hydraulic forging presswhich may be used for carrying out the cold forging of the invention;and

FIGURE 3 depicts diagrammatically the residual stresses present in ahand forging before and after cold forging according to the invention.

Various mechanical stress relieving methods, that is, the reduction ofresidual stresses by mechanical methods at moderate temperatures, havebeen known in the art. The reduction of residual stresses isaccomplished by mechanically introducing stresses that cause plasticflow. A commonly used method of stress relieving involves stressing themetal stock beyond the yield point by pressing, drawing or stretching.This procedure is frequently combined with a straightening operation.Although pressing, drawing or stretching will reduce some of theresidual stresses present in the metal, it is not as effective as thecold forging method with which this invention is concerned. Also, stressrelieving methods involving stretching etc. are not readily adaptablefor use with some fabricated parts, such as hand forgings.

The method of this invention involves the elimination or substantialreduction of residual quenching stresses in forgings by a cold forgingoperation which produces a non-uniform plastic strain across thethickness of the stock, the plastic strain being highest toward thecenter and zero at the" surfaces. In the cold forging operation,

two important factors control the character of the plastic strainpattern. The ratio of stock thickness or height to forging step widthmay alter the shape of the plastic strain pattern, while the percentagereduction could change about 2.0 to 2.5. The reduction of the stockthickness by the cold forging operation should be from about 1% to 4%,preferably 2%.

The residual elastic strains or stresses after springback are tensile atthe surface and compressive at the center. Both the plastic strains andelastic springb-ack strains are shown schematically in FIGURE 1,together with the effect of forging step width upon the strain pattern.It willbe noted from FIGURE 1 that the residual elastic stress patterncan be varied from one with a sharp peak at the center to one with a Wshape, the latter being the result of a narrow forging step width. Thiscontrasts with a stretching operation which produces a uniform plasticstrain across the thickness. -In a stretching operation after release ofa load, there is no residual elastic stress afiter spring-back 'if'there were none before stretch.

The residual stress caused by quenching is compressive at the surfaceand tensile at the center; that is, opposite in sign to that caused bycold forging. Consequently, by use of the proper forging step widthrelative to the thickness of the stock, it is possible to shape thecurve of residual stress caused by cold forging to conform very closelyto that caused by quenching, but opposite in sign; The magnitude of theresidual stress should then be adjusted by the correct amount ofpercentage reduction. By cold forging it is possible to reverse theinternal stress pattern caused by quenching. In a stretching operationfor stress relief, it is only possible to approach Zero stress by themaximum possible stretch. The sign of the stress can never be reversed.The cold forging method of stress relief permits a wider latitude ofcontrol of the final stress pattern.

The following examples, which are not intended to limit the invention,are illustrative of the eflicacy of the instant invention in relievingresidual stresses caused by quenching of hand forgings of solution heattreatable aluminum alloys by cold forging the said hand forgings acritical amount subsequent to the quenching step.

Example I A hand forging of aluminum alloy 7075 (having a nominalcomposition of 1.6% copper, 2.5% magnesium, 5.6% zinc, 0.3% chromium,balance aluminum and normal impurities), 3.31 inches by 9 inches by 21inches, was sawed into 2 equal sections, each section measuring 3.31inches by 9 inches by 10.5 inches. Both sections were solution heattreated at 865 F. for 6 hours and quenched in water at 140 F. Afterquenching, one piece was reduced 3.1% in thickness by cold forging on ahydraulic press as shown in FIGURE 2. The apparatus generally compriseda top plate '1 with a top die or tool engaged therewith by means of diekey 9 and a base plate 3 with a bottom die or tool 7' engaged therewithby means of die key 11. In FIGURE 2 the hand forging is designated as Fand the direction of working is shown by an arrow. The working surfaces15 and 17 of top die 5 and bottom die 7 respectively are shown beveledalong their long dimensions. The width of the die or tool which is usedin determining the ratio of stock thickness to forging step width is thewidth of the working surfaces 15 and 17 exclusive of the beveledsurfaces. Stop or gauge blocks 13 are disposed between top die 5 andbottom die 7 for controlling the reduction of thickness of the stock. 7

The width of the dies used in this example was 6 inches and the originalthickness of the forging was 3.31 inches. The stop or gauge blocks were3.19 inches in length. Allowances of 0.004 inch per inch of thicknesswere made for the expected elastic spring-back. There was a 0.5 inchoverlap of the width of the dies between successive forging steps;therefore the forging step width was 6-0.5 or 5.5 inches. The ratio,then, of the thickness of the hand forging to the forging step width was3.31/5.5 or 0.6. After the one piece had been cold worked, both pieceswere artificially aged for 24 hours at 250 F.

The effect of the stress relief was determined by sawing the pieces inhalf parallel to the 9 inch by 10.5 inch faces of the sections andmeasuring the distortion for both the longitudinal dimensions (directionof working) and the transverse dimensions.

In the case of the unrelieved specimen, the distortion in thelongitudinal dimension (10.5 inch dimension) was 0.068 inch or 0.013inch/inch, and in the transverse dimension (9 inch dimension) thedistortion was 0.045 inch or 0.01 inch/inch. In the case of the specimenwhich was stress relieved according to the invention, there was nomeasurable warpage or distortion in the longitudinal dimension (10.5inch dimension) while the distortion in the transverse dimension (9 inchdimension) was only 0.014 inch or 0.003 inch/inch.

Example II Two hand forgings of 7075 aluminum alloy were solution heatedat 865 F. for six hours and quenched in water at 140 F. The dimensionsof each forging were 3 inches by 10 inches by 36 inches. One of thequenched forgings was reduced, 2% in thickness by cold forging on ahydraulic press shown in FIGURE 2. The forging step width was 1.25inches. The ratio of the forging thickness to the forging step width was3/ 1.25 or 2.4. After the cold forging operation performed on the oneforging, both forgings were artificially aged for 24 hours at 250 F.FIGURE 3 shows the residual stresses before and after cold forging, aswell as the plastic strains produced during cold forging. The left handdiagram of FIGURE 3 shows the measured residual stresses in the forgingwhich had been heat treated, quenched and aged without further work. Theright hand diagram shows the internal residual stress pattern throughthe thickness of the forging which was solution heated, quenched, coldforged and then aged. The center diagram shows the plastic strainpattern across the thickness of the forging effected by the cold forgingof this invention. Both the longitudinal and transverse stresses areshown in each diagram. It will be noted that the signs of the stressesin the longitudinal direction have actually been reversed, as well assubstantially reduced, and in the transverse direction the stresses havebeen reduced.

Various changes or modifications may be made without departing from thespirit and scope of the invention and, accordingly, the invention is notto be limited except by the appended claims wherein what is claimed is:

1. A method for producing forged shapes of solution heat treatablealuminum alloys wherein the shape is fabricated by forging, solutionheating, quenching and aging, the improvement for substantially reducingresidual stresses created by said quenching comprising the step of coldforging the said shape after quenching and prior to aging, said coldforging effecting a small reduction of thickness of said shape, theratio of the thickness of the shape to the forging step width used insaid cold forging step being in the range of about 0.3 to 4.0.

2. A method according to claim 1 wherein the ratio of the thickness ofthe shape to the forging step width is from about 2.0 to 2.5.

3. A method for producing forged shapes of solution heat treatablealuminum alloys wherein the shape is fabrica'ted by forging, solutionheating, quenching and aging, the improvement for substantially reducingresidual stresses created by said quenching comprising the step of coldforging the said shape after quenching and prior to aging to a reductionof from 1 to 4% of the total thickness of said shape, the ratio ofthickness of said shape to the forging step width used in the coldforging step being from about 0.3 to 4.0.

4. A method according to claim 3 wherein the said reduction is 2% of thetotal thickness of said shape and the ratio of the thickness of saidshape to the forging step width is from about 2.0 to 2.5.

5. A solution heated, quenched and aged forged article of a solutionheat treatable aluminum alloy which has been made according to themethod of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS1,891,234 Langenberg Dec. 20, 1932 2,454,312 Fritzlen Nov. 23, 19482,743,516 Forrester May 1, 1956 2,894,421 Appel July 14, 1959 FOREIGNPATENTS 423,868 Great Britain Feb. 11, 1935 738,070 Great Britain Oct.5, 1955

1. A METHOD FOR PRODUCING FORGED SHAPES OF SOLUTION HEAT TREATABLEALUMINUM ALLOYS WHEREIN THE SHAPE IS FABRICATED BY FORGING, SOLUTIONHEATING, QUENCHING AND AGING, THE IMPROVEMENT FOR SUBSTANTIALLY REDUCINGRESIDUAL STRESSES CREATED BY SAID QUENCHING COMPRISING THE STEP OF COLDFORGING THE SAID SHAPE AFTER QUENCHING AND PRIOR TO AGING, SAID COLDFORGING EFFECTING A SMALL REDUCTION OF THICKNESS OF SAID SHAPE, THERATIO OF THE THICKNESS OF THE SHAPE TO THE FORGING STEP WIDTH USED INSAID COLD FORGING STEP BEING IN THE RANGE OF ABOUT 0.3 TO 4.0.